1. Field of the Invention
The present invention relates to methods for imprinting micro-patterns, and relates in particular to a technique of evanescent(proximity)-field-assisted fabrication of two-dimensional micro-patterns of smaller dimensions than the wavelength of light.
2. Description of the Related Art
A typical example of fabrication techniques for making micro-circuit patterns on a semiconductor substrate is photolithography. In this technique, a photo-sensitive material (photo-resist) is coated on a substrate base, and a reduced image of a masking pattern containing micro-circuit patterns is projected on the base by optical means to expose the photo-resist material. The degree of resolution achievable by photolithography is limited by light diffraction effects, and the minimum line width is ultimately limited by the wavelength with respect to light. For this reason, it is necessary to use shorter wavelengths for fabrication of finer patterns. Currently, this is being achieved by using the g-line (436 nm wavelength) or the i-line (365 nm wavelength) of mercury lamp, or KrF excimer laser (248 nm wavelength) or ArF excimer laser (193 nm wavelength), and the trend is towards the use of shorter wavelengths to meet the demand for finer line width.
In recent years, active research has been conducted on the use of evanescent field (proximity field) for fabrication of micro-patterns. An evanescent field is an electromagnetic field produced when light is transmitted through a transmissive object placed at a sub-wavelength distance of a light source. If an object having a micro-pattern surface structure, comprised by high and low structures, is placed within a sub-wavelength of a light source, light is transmitted through the high structures of the structure, and evanescent fields are generated at these locations. Evanescent field diminishes exponentially as the separation distance increases beyond the wavelength of transmitting light. Therefore, if a micro-pattern with height difference of the order of several tens of nanometers is coated with a photo-sensitive material and is placed at a sub-wavelength distance to an exposure light source, light is transmitted only through the high structures, thereby exposing only the coating on high structures of the micro-pattern to the exposure light. In this technique, the line width of the exposed material is governed only by the fineness of the pattern placed in proximity of the light source, not by the exposure wavelength. Therefore, it is possible to produce micro-patterns exceeding the limit imposed by the exposure wavelength.
A known example of micro-pattern fabrication based on the evanescent field effect uses an optical fiber having one end sharpened to a sub-wavelength dimension, and a laser light is injected from the opposite end of the fiber. The sharpened end is placed in contact with or in proximity to (at a sub-wavelength distance) a surface of a substrate base coated with a photo-resist film, then a proximity field is produced in the vicinity of the exposed region, and the light is transmitted through the proximity field and the photo-resist film is exposed to the light. Therefore, by sharpening the tip to a sub-wavelength dimension, it is possible to expose a pattern scribed by a line width of a sub-wavelength dimension on the photo-resist film. The photo-resist film is developed by photo-lithographical technology, then, using the exposed sections of the photo-resist film as the etch-masking, patterned surface of the substrate base is etched to remove the unprotected regions, thereby leaving behind micro-patterns of a sub-wavelength line width.
However, in this technique, the proximity field can only be produced at the tip of the optical fiber opposite to the base, therefore, the exposed pattern is a point. To apply this technique to the production of a two-dimensional pattern of some integrated circuit device, it is necessary to scan the tip in a pattern of the circuit, so that it not only consumes a vast amount pattern-making time but also leads to the need for a complex tip-driving apparatus. Therefore, this technique is considered impractical.
For these reason, there have been attempts to produce micro-patterns using a mask that has a proximity field exposure pattern and transferring the two-dimensional pattern to a substrate base. For example, a prism made of a light transmissive material such as glass and the like is prepared and a photo-mask, having a proximity field exposure pattern of sub-wavelength dimensions, is attached to the bottom surface of the prism. Light is injected into the prism at such an angle that it is totally reflected at the bottom surface of the prism. Next, a substrate base coated with a photo-resist film is placed at a sub-wavelength distance of a proximity exposure pattern so that an evanescent field is produced and a two-dimensional pattern, conforming to the proximity exposure pattern, is exposed on the photo-resist film. In this process, an optical system for injecting a laser beam from an inclined surface of a prism is used, and the incident laser beam is totally reflected at a plane having a proximity field exposure pattern; and then it is transmitted to outside through another inclined surface. A photo-resist film surface of a substrate base is made to contact closely with the proximity field exposure pattern, so as to generate an evanescent field to propagate exposure light along the exposure pattern, and thereby producing micro-patterns having line widths of sub-wavelength dimensions.
The method utilizing the evanescent field described above enables to produce a two-dimensional micro-pattern having line widths less than the wavelength of the exposure light on a photo-resist film of a substrate. However, the method requires that the incident beam be aligned with the inclination angle of a prism, and the optical system is necessarily complex. Also, the exposure pattern section can only accept a small exposure area. And, because the incident light is at an angle to the proximity field exposure pattern, the depth of imprinting is shallow, and because the exposed area increases quickly along the beam line, it is difficult to expose a structure having high aspect ratios on the photo-resist film.
It is an object of the present invention to provide an optical imprinting apparatus and method for producing a two-dimensional pattern, having line widths less than the wavelength of an exposure light, economically using a simple method and a low cost device.
It is another object of the invention to provide an imprinting method for imprinting two dimensional micro patterns with high aspect ratio.
In order to achieve the above object, there is provided an apparatus, which comprises a waveguide; a proximity field exposure pattern firmly fixed to a section of the waveguide; and a light source for injecting a light into the waveguide.
According to the present invention, by placing a substrate surface close to the proximity field exposure pattern, a portion of the light propagating in the waveguide is transmitted through an evanescent field to expose the substrate surface. Therefore, a compact apparatus having a waveguide having a proximity field exposure pattern and a light source is provided, which is much more compact compared with the conventional stepper device, and is much more economical. Because the waveguide can be made in a flat shape, by combining with a compact semiconductor laser and the like, the overall imprinting apparatus can be made quite compact.
An optical imprinting apparatus is comprised by a light source; and an exposure-mask having a proximity field exposure pattern for radiating an exposure light output from the light source on a photo-sensitive material through an evanescent field; wherein the exposure-mask is provided integrally with the light source.
The apparatus having an integral light source and the proximity field exposure pattern can provide a very compact apparatus, and can eliminate complex optical systems required in the conventional optical systems and produce micro-patterns of sub-wavelength dimensions.
A method for evanescent-field-assisted imprinting is comprised by the steps of: placing a proximity field exposure pattern on a waveguide or on a portion of a light source; aligning a fabrication object having a photo-sensitive film in proximity of the proximity field exposure pattern; injecting a light from the waveguide or the light source into the proximity field exposure pattern so as to imprint the proximity field exposure pattern on the photo-sensitive material by means of an evanescent field formed between the proximity field exposure pattern and the photo-sensitive film.
Because the proximity field exposure pattern is extremely close to the photo-sensitive material, micro-patterns are imprinted by the evanescent field effect to imprint two dimensional patterns using a simple apparatus. Direct use of the light source enables total reflection in a simple manner and a large area can be exposed very uniformly to provide a very effective process.
An exposure-mask for imprinting micro-patterns on a mask base is made in cooperation with an evanescent field generated by exposure light output from a light source, wherein the mask base is transmissive to the exposure light and is provided with micro-patterns comprised by high structures and low structures of sub-wavelength dimensions with respect to a wavelength of the exposure light, and the low structures are embedded with a material of low transmissivity to the exposure light.
The proximity exposure enables micro-patterns to be exposed on a photo-sensitive material, and the low structures to be filled with a low transmissivity material, so that there is no light leaking from such regions of the patterns. The evanescent fields are produced only on the high structures, and a high contrast (large height differences in the micro-pattern) can be produced in the pattern.
It is preferable that the low transmissivity material be a metallic substance. Such a structure can be produced by vapor deposition or sputtering of a metal film, followed by chemical mechanical polishing to flatten the surface of the photo-sensitive material to produce an excellent photo-mask.
The low transmissivity material may be produced by ion exchange. It is also preferable that fabrication of the mask base is performed by fast atomic beams.
A method for making an exposure-mask having a fine pattern is comprised by the steps of: applying a photo-sensitive coating on a mask base made of a material transmissive to exposure light; fabricating micro-patterns on the photo-sensitive coating using electron beams or X-ray beams; and irradiating with a fast atomic beam using the micro-patterns fabricated on the photo-sensitive film as exposure-mask; thereby imprinting micro-patterns on the mask base.
Micro-patterns can be produced on quartz glass or other transmissive materials together with a photo-sensitive coating and exposure by electron beam or X-rays. Thickness of the photo-sensitive material should be up to twice the size of the micro-patterns. FAB is linear and free of charge accumulation so that insulation materials can be fabricated with micropatterns of high aspect ratios. Parallel plate type FABs are preferable for use here. Also, it is noted that this technique is also applicable to the pattern dimensions of larger than wavelength of the light.
A method for imprinting micro-patterns on a substrate base is comprised by the steps of: applying not less than two layers, including an upper layer of a photo-sensitive film having a thickness dimension of less than the wavelength of exposure light; placing an exposure-mask having proximity patterns in contact with or in proximity of the photo-sensitive film at a sub-wavelength distance so as to generate an evanescent field and expose the proximity patterns on the photo-sensitive film; developing exposed proximity patterns by photo-processing to produce a first etch-mask; fabricating a lower surface of the substrate base using the first etch-mask to produce a second etch-mask comprised by the lower film; and imprinting proximity field exposure patterns on the substrate base using the second etch-mask.
It is again demonstrated that the evanescent field effect promotes production of sharp micro-patterns. The depth of patterns produced by this technique is shallow, and the exposed areas are dispersed quickly along the depth direction. Therefore, thin or film is able to perform precise fabrication.
A method for imprinting micro-patterns on a substrate base is comprised by the steps of: applying a first coating of a photo-sensitive material on the substrate base to a thickness less than a wavelength of an exposure light; placing an exposure-mask having a proximity field exposure pattern in contact with or in proximity of the proximity field exposure pattern at a sub-wavelength distance; exposing the coating through the exposure-mask using the exposure light through an evanescent field and developing by photo-processing to produce first imprinted patterns on the first coating; applying a second coating on the first imprinted pattern of the photo-sensitive material; dissolving the first coating to liftoff the first imprinted patterns, thereby leaving second imprinted patterns formed by the second coating; and fabricating the substrate base using the second imprinted patterns as etch-mask to produce micro-patterns on the substrate base.
Using such an exposure-mask, thick film as a second coating can also be imprinted to produce imprinted micro-patterns. Thick film has excellent tolerability so that high aspect structures can be produced readily by using the etch-mask technique with thick films.
It is preferable that the thickness of the first coating is essentially the same as a minimum dimension of the proximity field exposure pattern. It enables sharp imprinting of minimum dimensions. It is preferable that the fabrication of the substrate base or an exposed lower layer is performed using a fast atomic beam. FAB has highly linear etching property and free from charge accumulation so that micro-patterns of high aspect ratios can be produced efficiently.
An exposure-mask for evanescent-field-assisted imprinting having a proximity field exposure pattern of sub-wavelength dimensions is fabricated on a transmissive material, wherein the proximity field exposure pattern is produced by imprinting a master proximity field exposure pattern provided on a mother mold.
This technique is suitable for mass production of proximity field exposure patterns at low cost. It can eliminate the necessity for expensive original mask for evanescent field assisted imprinting which can be applied to the production of LSI circuits.
It is preferable that the mother mold is a metal mold.
Metal mold can easily be detached from glass or resins so that the mask can be preserved well. Also, metals are excellent for producing micro-patterns and for imprinting purposes.
A method for making an exposure-mask is by preparing a mother mold having a pattern; pouring a transmissive material in a molten state into the mother mold; cooling and detaching a solidified pattern from the mother mold, thereby producing an imprinted proximity field exposure pattern.
It is therefore possible to use low cost materials such as plastics, resins (PMMA) to produce micro-patterns, and the cost of making exposure-mask is reduced.
It is preferable that detaching from the mother mold is based on differential thermal expansion effects of materials constituting a mother mold and an imprinted pattern.
This separation technique is enhanced by a metal mold when glass and resins are used for making duplication mask.
It is preferable that the mother mold is pre-coated with a soluble film, which is dissolved when detaching a solidified pattern from the mother mold.
This technique is cost effective and can easily be applied to many applications when making duplication mask.
The base may be coated with SiO2 followed by a resist film to make a fine pattern, which is used as mask to etch the SiO2 film with FAB, and a metal film is further deposited, and the SiO2 film is removed by dissolving in HF solution. The metal film can be used as a mother mold.
A method for imprinting micro-patterns on an imprint base by preparing a pattern template having a fine structure, coating a semi-solid material on the pattern template; pressing the semi-solid material on the pattern template to produce a duplicated pattern of the fine structure; irradiating the duplicated pattern to produce the imprint base having the micro-patterns.
This technique eliminates the need for alignment of micro-patterns and expensive optical equipment is not needed, so that low cost mass production of micro-patterns is made possible. Also, it is noted that this technique is also applicable to the pattern dimensions of larger than wavelength of the light.
It is preferable that the pattern template is a roller having the fine-structure fabricated on an roller surface, and the fine-structure is duplicated on an imprint base by press rolling on a semi-solid material.
This technique is quite adaptable and can be applied to a curved surface.
It is preferable that the pattern template is a flexible material disposed away from a semi-solid imprint base, which is roll pressed by a roller template to intimately contact the roller template and thereby imprinting the micro-patterns on the semi-solid imprint base.
This technique is applicable to non-flat surfaces to imprint micro-patterns accurately.
A method for imprinting micro-patterns on an imprint base by preparing a pattern template having a fine structure comprised of high and low structures, pouring a molten material on the pattern template; cooling the molten material on the pattern template; detaching a solidified pattern to produce a duplicated pattern of the fine structure to produce the imprint base having the micro-pattern.
This technique allows pouring of molten material to produce an imprint base having micro-patterns so that it is effective for low cost mass production.
A method for fabricating micro-patterns on an imprint base is by applying a photo-resist layer coating on a pattern template; scribing a fine structure on the photo-resist coating by means of electron beams or X-ray beams and developing by photo-processing to fabricate etch-mask; irradiating with a fast atomic beam through the etch-mask to produce an imprint base having the fine structure duplicated thereon.
Electron or X-ray beams do not present diffraction problems so that micro-patterns can be produced on resist film easily, which is used as etch-masking for FAB to produce micro-patterns of high aspect ratios on imprint bases. Also, it is noted that this technique is also applicable to the pattern dimensions of larger than wavelength of the light.
A method for imprinting on an imprint base for LSI devices is by preparing an exposure-mask having a fine structure of sub-wavelength dimensions; exposing a substrate base of a semiconductor material coated with a photo-sensitive material through the exposure-mask in an evanescent field so as to imprint the fine structure on the LSI device base.
This method allows micro-patterns to be imprinted on photo-resist film easily and effectively. The exposure apparatus is comprised only by a mask having a proximity field exposure pattern, eliminating the need for complex optics. The apparatus cost is low and the production cost is low.
A method for imprinting micro-patterns on LSI devices is by preparing a pattern template having a fine structure of sub-wavelength dimensions; pressing the pattern template on a semi-solid material coated on a substrate base of a semiconductor material so as to imprint the fine structure on a surface coating; and etching the surface coating using imprinted patterns as etch-mask to produce the LSI devices.
Imprint template in this case corresponds to normal photo-mask used in fabrication of LSI devices, but the template has micro-patterns which can be replicated readily by pressing on semi-solid materials. The imprinted patterns are used as etch-mask for FAB to produce micro-patterns of high aspect ratios on an LSI base.
An optical data recording medium contains recorded signals fabricated using a method of evanescent-field-assisted fabrication.
Signal bits produced by the evanescent field effects has micro-patterns of sub-wavelength dimensions, so that the density of recording can be increased in quantum steps.
An optical data recording apparatus is comprised by: a recording medium having micro-patterns of sub-wavelength dimensions, with respect to signal light, having different transmissive and reflective properties; a light source for signal light; and a detection section disposed opposite to a patterned surface of the recording medium.
The data recording medium can store significantly more data compared with similar conventional recording medium. The apparatus is a stationary recording apparatus of an extremely overall compact size. No moving parts are involved in such an apparatus, so that potential wear is reduced and accompanying operating noise is minimal.
An magnetic-optical recording head is comprised by an optical fiber having a sharpened tip of a sub-wavelength dimension with respect to signal light, and a magnetic field generation coil for magnetizing a magnetic layer disposed in proximity of the sharpened tip in association with the magnetic-optical recording head.
Signal transfer is carried out through the evanescent field at the nano-tip of the fiber, so that data transfer, in association with the magnetic field produced in the head and storage medium of micro-sized dimensions, to lead to a quantum increase in data density and performance speeds.